Re: intel_iommu unit test is also failing

[Thomas Huth <thuth@redhat.com>]

On 05/05/2026 12.53, Michael S. Tsirkin wrote:
> On Tue, May 05, 2026 at 12:34:41PM +0200, Thomas Huth wrote:
>> On 05/05/2026 12.23, Michael S. Tsirkin wrote:
>>> On Tue, May 05, 2026 at 11:45:17AM +0200, Thomas Huth wrote:
>>>> On 05/05/2026 11.27, Clément MATHIEU--DRIF wrote:
>>>>> I had a bit more time to hook into qemu to check the root cause.
>>>>>
>>>>> It seems that testb issues a single byte read (out of the valid size range), as we can see on the following breakpoint:
>>>>>
>>>>> ```
>>>>> Thread 6 "CPU 0/TCG" hit Breakpoint 2, memory_region_dispatch_read (mr=0x55d72883cb30, addr=152, pval=0x7f62d25f4590, op=MO_BSWAP, attrs=...) at ../system/memory.c:1473
>>>>> 1473       unsigned size = memop_size(op);
>>>>> (gdb) n
>>>>> 1474       MemTxResult r;
>>>>> (gdb) p size
>>>>> $1 = 1
>>>>> (gdb)
>>>>> ```
>>>>
>>>> Ouch! That's an excellent finding, Clément ... so GCC 16 is "smart" enough
>>>> to see that we only want to test the lowest bit here, so it optimizes the
>>>> code to access only one byte of memory instead of 4 bytes... which would be
>>>> ok for normal memory, but not for an MMIO register :-/
>>>>
>>>> Ugly work-around, to force GCC to read 32 bits:
>>>>
>>>> diff --git a/lib/asm-generic/io.h b/lib/asm-generic/io.h
>>>> --- a/lib/asm-generic/io.h
>>>> +++ b/lib/asm-generic/io.h
>>>> @@ -38,7 +38,9 @@ static inline u16 __raw_readw(const volatile void *addr)
>>>>    #ifndef __raw_readl
>>>>    static inline u32 __raw_readl(const volatile void *addr)
>>>>    {
>>>> -       return *(const volatile u32 *)addr;
>>>> +       u32 val = *(const volatile u32 *)addr;
>>>> +       asm volatile ("\n" : : "r"(addr));
>>>> +       return val;
>>>>    }
>>>>    #endif
>>>>
>>>> ... but I wonder whether this should rather be treated as a bug in GCC
>>>> instead, since it should IMHO really not change the access size for a
>>>> volatile memory access?
>>>>
>>>>    Thomas
>>>
>>> Wouldn't this break linux generally?
>>>
>>> #ifndef __READ_ONCE
>>> #define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))
>>> #endif
>>
>> I asked myself the very same question, but after googling for "GCC 16 linux
>> kernel" issues, I did not find anything related... there is likely something
>> specific to kvm-unit-tests in here...
>>
>>   Thomas
> 
> 
> This seems to be pertinent:
> https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
> 
> -ffuse-ops-with-volatile-access
> Allow limited optimization of operations with volatile memory access when doing so does not change the semantics outlined in See When is a Volatile Object Accessed?.
> 
> The default is -ffuse-ops-with-volatile-access
> 
> implemented here:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122343
> 
> Try disabling? -fno-fuse-ops-with-volatile-access

Thanks, this seems to fix the issue, indeed!

Would you like to send a patch for it?

  Thomas